eV stands for electron volt--that is, the amount of energy needed to
move one electron through a potential difference of one volt. If that doesn't
mean much to you, don't worry about it; the point is that it's a small unit of
energy, convenient for the scale of energy levels in an atom.
To give you an idea of just how small an eV is...a 100 watt light bulb is putting out 100 Joules of energy per second. That's about 625 quintillion (625,000,000,000,000,000,000) eV per second!
You may also run across KeV (kilo-eV; that's one thousand eV) and Mev
(mega-eV, which is one million eV).